1. Field of the Invention
This invention relates generally to a semiconductor laser, and more particularly, to a semiconductor laser incorporating an array of coherently coupled semiconductor laser array cells on a common substrate.
2. Discussion of the Related Art
Semiconductor lasers are well known in the art. These types of lasers have a number of desirable features including compactness of size, narrow bandwidth, reliability, and a highly directional light beam. It is therefore desirable to utilize semiconductor lasers for a variety of applications including certain medical applications, optical welding, optical storage and signal processing, laser range finding and free-space communications.
By way of background information, the basic operation of a conventional semiconductor laser will be described. As is generally known in the art, a semiconductor laser typically incorporates a multi-layered structure comprised of a number of specially doped semiconductor regions. The composition of the semiconductor material of the different layers is determined to provide a discrimination in the indices of refraction between at least some of the different layers, and thus develop a layer in which light can be confined. Adjacent layers to the confining layer, or active region, are typically referred to as cladding layers. A forward bias current across the confining layer will enable the charge carriers to recombine and emit light at a particular frequency to provide the lasing action. By confining the lasing region of the semiconductor laser to the active region, it is possible to increase the intensity of this generated light. By incorporating mirrors at each end of the active region, waves of light will oscillate back and forth within this region such that the intensity of the wave will increase until the radiation generates enough energy for lasing to occur within the semiconducting material.
Not only must the light be confined in the active region in a vertical direction by means of the layer structure, it must also be confined in the active region in a lateral direction. One known method of confining the light in a lateral direction within a particular active region is generally referred to as positive index guiding. In this type of laser, the refractive index of the material is highest in regions in which the light propagates and lower in regions aligned with the higher index of refraction regions. Another method of confining the light is known as negative-index guiding. In a negative-index guiding or antiguide laser, the refractive index of the semiconductor material is lowest in the regions in which the light propagates and higher in adjacent aligned regions. Consequently, some of the light which is incident on the higher refractive index boundary would leak out of the lasing region.
In a conventional semiconductor laser, propagation of the light is generally limited to a single region or element within the semiconductor. Because of this the power output of the laser is limited. To increase the power output of semiconductor lasers, it is known to fabricate arrays for which the elements are coupled to each other.
One known antiguide semiconductor laser array is the phase-locked, resonant optical waveguide (ROW) which incorporates an array of elements resonantly coupled to each other by leaky waves. See for example, Botez et al., Phase-Locked Arrays of Antiguides: Modal Content and Discrimination, "IEEE Journal of Quantum Electronics", Vol. 26, No. 3, Mar. 1990 and Mawst et al., High-Power, Narrow-Lobe Operation from 20-Element Phase-Locked Arrays of Antiguides, "Appl. Phys. Lett.," 55 (20), 13 Nov. 1989, both of which are herein incorporated by reference. In these types of ROW lasers, an array cell of aligned channels are fabricated on a common substrate by an appropriate semiconductor deposition process, such as metal organic chemical vapor deposition (MOCVD), to form an alternating array of elements having a first index of refraction adjacent interelements having a higher index of refraction. A portion of the laser radiation propagating in an active region within each of the elements will be leaked into the interelements at each reflection off the walls of the elements at the transition of low to high indices of refraction. The laser radiation which is leaked out of one element, crosses the adjoining interelement and enters an adjacent element, and is coherently coupled to the radiation traveling in that element depending on the distance between the adjacent elements, thus creating a resonance condition. Since the leaked radiation from one element is coupled with the radiation in the next element, it adds to the intensity of the laser radiation in that element. Once the intensity of the laser radiation reaches a lasing threshold level, and all elements are coupled in phase with each other, light from the semiconductor array is emitted into a narrow beam at an angle substantially perpendicular to the plane of the substrate. A higher number of elements thus results in a higher intensity beam.
For an array cell of laser elements as discussed above, it is possible that the radiation being reflected back and forth oscillates in more than one spatial mode. For an array cell of lasers which operates in several modes, the generated laser beam will have many uncorrelated lobes. The array cell can be thought of as emitting more than one beam for each different mode. The quality of the semiconductor laser, expressed as the Strehl ratio, is reduced due to the increased width of the beam distribution pattern. It is desirable in most applications to produce a semiconductor laser which operates in a single mode. The single fundamental mode preferably should be one in which all of the elements are in phase, called the in-phase mode, to achieve a high Strehl ratio.
Strong coupling of leaked radiation between the elements lends itself to the suppression of other, secondary modes of radiation having a separate phase than the primary or in-phase mode. Mode discrimination is partly realized due to the presence of transverse losses within the interelements. However, as the width of the array increases due to an increase in the number of coupled elements, the more difficult it is to maintain a single mode, and thus, the number of modes may increase above the fundamental mode. In addition, the photolithographic and deposition processes used to fabricate the array also limits the dimensions of each of the elements, and interelements. With the present technology, a semiconductor array cell of this type is limited to about 40 different adjoining array elements having a combined width of about 200 microns. An array cell of a wider dimension generally results in adverse transmission of secondary modes which interfere with the primary in-phase mode of the laser beam. Consequently, the power output of the semiconductor laser is limited by these factors. For a more thorough discussion of modal discrimination in a semiconductor laser array cell of the type discussed above, see Botez et al U.S. Pat. No. 5,063 570, issued Nov. 5, 1991, herein incorporated by reference and assigned to the assignee of the instant application.
What is needed then is a semiconductor laser incorporating multiple arrays of antiguides which has an increased power output over the prior art semiconductor lasers, and whose output is limited to a single mode. It is therefore an object of the present invention to provide appropriate coupling of such semiconductor lasers in a two-dimensional configuration.